The interaction of O₃ and CO₂ concentration, exposure timing and duration on stem rust severity on winter wheat...

Wheat rusts, elevated ozone (O₃), and carbon dioxide (CO₂) are simultaneously impacting wheat production worldwide, but their interactions are not well understood. This study investigated whether near-ambient O₃ is suppressive or conducive to stem rust (Sr) of wheat, considering the interactions with ambient and elevated CO₂. Winter wheat variety 'Coker 9553' (Sr-susceptible; O₃ sensitive) was inoculated with Sr (race QFCSC) following pre-treatment with four different concentrations of O₃ (CF, 50, 70, and 90 ppbv) at ambient CO₂ levels. Gas treatments were continued during the development of disease symptoms. Disease severity, measured as percent sporulation area (PSA), significantly increased relative to the CF control only under near-ambient O₃ conditions (50 ppbv) in the absence of O₃-induced foliar injury. Disease symptoms at higher O₃ exposures (70 and 90 ppbv) were similar to or less than the CF control. When Coker 9553 was inoculated with Sr while exposed to CO₂ (400; 570 ppmv) and O₃ (CF; 50 ppbv) in four different combinations, and seven combinations of exposure timing and duration, PSA significantly increased only under continuous treatment with O₃ for six weeks or pre-inoculation treatment for three weeks, suggesting that O₃-predisposes wheat to the disease rather than enhancing disease post-inoculation. O₃ singly and in combination with CO₂ increased PSA on flag leaves of adult Coker 9553 plants while elevated CO₂ alone had little effect on PSA. These findings show that sub-symptomatic O₃ conditions are conducive to stem rust, contradicting the current consensus that biotrophic pathogens are suppressed by elevated O₃. This suggests that sub-symptomatic O₃ stress may enhance rust diseases in wheat-growing regions.